A 36-year-old Jersey City man who is a registered sex offender was arrested Monday after he was caught masturbating in his car outside Midtown Community School in Bayonne, police said yesterday.

Christopher Arnold, 36, of Magnolia Avenue in Jersey City, was charged with lewdness and sexual assault following the incident, police said.

Patrick Sullivan, an off-duty Jersey City police sergeant who was dropping off his children at the Avenue A school around 8:40 a.m. Monday was approached by a local resident who said he saw a man, later identified as Arnold, masturbating inside his vehicle, police said.

After checks following his arrest, police discovered that Arnold was on the registered sex offenders list, police said. According to the New Jersey State Police Megan’s Law website, Arnold was found guilty in May 2004 of sexual assault and endangering the welfare of a child after he sexually assaulted two underage girls, 14 and 15 years old.

Yesterday, Arnold’s bail was set by Hudson County Superior Court Judge Fred Theemling at $1 million and he was detained at the Hudson County jail in Kearny, police said.

He was charged with sexual assault in yesterday’s case because his actions could have been seen by children under 13 years old, authorities said.